This invention relates generally to optical communications networks and, more particularly, to methods and systems for providing long haul optical communication systems that employ Raman amplification.
From the advent of the telephone, people and businesses have craved communication technology and its ability to transport information in various formats, e.g., voice, image, etc., over long distances. Typical of innovations in communication technology, recent developments have provided enhanced communications capabilities in terms of the speed at which data can be transferred, as well as the overall amount of data being transferred. As these capabilities improve, new content delivery vehicles, e.g., the Internet, wireless telephony, etc., drive the provision of new services, e.g., purchasing items remotely over the Internet, receiving stock quotes using wireless short messaging service (SMS) capabilities etc., which in turn fuels demand for additional communications capabilities and innovation.
Recently, optical communications have come to the forefront as a next generation communication technology. Advances in optical fibers over which optical data signals can be transmitted, as well as techniques for efficiently using the bandwidth available on such fibers, such as wavelength division multiplexing (WDM), have resulted in optical technologies being the technology of choice for state-of-the-art long haul communication systems.
For long haul optical communications, e.g., greater than several hundred kilometers, the optical signal must be periodically amplified to compensate for the tendency of the data signal to attenuate. For example, in the submarine optical communication system 10 shown in FIG. 1, the terrestrial signal is processed in WDM terminal 12 for transmission via optical fiber 14. Periodically, e.g., every 75 km, a line unit 16 amplifies the transmitted signal so that it arrives at WDM terminal 18 with sufficient signal strength (and quality) to be successfully transformed back into a terrestrial signal.
Conventionally, erbium-doped fiber amplifiers (EDFAs) have been used for amplification in the line units 16 of such systems. As seen in FIG. 2(a), an EDFA employs a length of erbium-doped fiber 20 inserted between the spans of conventional fiber 22. A pump laser 24 injects a pumping signal having a wavelength of, for example, approximately 1480 nm into the erbium-doped fiber 20 via a coupler 26. This pumping signal interacts with the f-shell of the erbium atoms to stimulate energy emissions that amplify the incoming optical data signal, which has a wavelength of, for example, about 1550 nm. One drawback of EDFA amplification techniques is the relatively narrow bandwidth within which this form of resonant amplification occurs, i.e., the so-called erbium spectrum. Future generation systems will likely require wider bandwidths than that available from EDFA amplification in order to increase the number of channels (wavelengths) available on each fiber, thereby increasing system capacity.
Like other types of communication technologies which squeeze capacity from a finite amount of bandwidth, WDM optical communication systems must cope with the adverse effects of both noise and intersymbol interference (ISI). Noise tends to make the original signal more difficult to reproduce by adding extraneous energy to the original signal, whereas ISI tends to xe2x80x9csmearxe2x80x9d the original signal when, for example, signal energy originally transmitted on one wavelength channel bleeds into signal energy originally transmitted on another wavelength channel. With respect to EDFA amplification schemes, noise is a particular problem, since the EDFA amplifier inherently generates a significant amount of noise in the form of amplified spontaneous emission (ASE). ASE occurs because, in addition to generating energy that amplifies the incoming data signal, the decay of the erbium atoms creates light that is itself propagated through the optical fiber along with data signal. Since ASE is generated by each amplifier along the path between terminals, the ASE noise tends to accumulate. In some EDFA systems, the noise accumulates to a degree that the system design may be impacted. Consider, for example, the system described in the article xe2x80x9c1800 Gb/s Transmission of One Hundred and Eighty 10 Gb/sWDM Channels over 7,000 km using the Full EDFA C-Bandxe2x80x9d, by C. R. Davidson et al., found in OFC""2000, Paper PD25-1, the disclosure of which is incorporated herein by reference. There, an EDFA optical communication system was tested in a circulating loop and experienced unexpectedly large noise values near the 1532 nm gain peak. To offset the reduced performance caused by this unexpected noise, the channel density in that system was reduced in the vicinity of the accumulated noise.
Distributed Raman amplification is one amplification scheme that can provide a broad and relatively flat gain profile over a wider wavelength range than that which has conventionally been used in optical communication systems employing EDFA amplification techniques. Raman amplifiers employ a phenomenon known as xe2x80x9cstimulated Raman scatteringxe2x80x9d to amplify the transmitted optical signal. In stimulated Raman scattering, as shown in FIG. 2(b), radiation from a pump laser 24 interacts with a gain medium 22 through which the optical transmission signal passes to transfer power to that optical transmission signal. One of the benefits of Raman amplification is that the gain medium can be the optical fiber 22 itself, i.e., doping of the gain material with a rare-earth element is not required as in EDFA techniques. The wavelength of the pump laser 24 is selected such that the vibration energy generated by the pump laser beam""s interaction with the gain medium 22 is transferred to the transmitted optical signal in a particular wavelength range, which range establishes the gain profile of the pump laser.
Although the ability to amplify an optical signal over a wide bandwidth makes Raman amplification an attractive option for next generation optical communication systems, the use of a relatively large number of high power pump lasers (and other components) for each amplifier in a Raman system has hitherto made EDFA amplification schemes the technology of choice for long haul optical communication systems. Thus, not surprisingly, the resources employed to design Raman-amplified, long haul communication systems have languished relative to those employed for designing EDFA-amplified long haul systems.
However, as the limits of EDFA amplification are now being reached, recent efforts have begun to explore the design issues associated with supplementing, or replacing, EDFA amplification technology with Raman amplification technology. These efforts have identified ISI (in particular nonlinear effects) associated with optical fiber signal transmission as one of the primary limiters of capacity in systems employing Raman-amplification in wide bandwidth, WDM optical communication systems.
Accordingly, there remains a need for techniques and systems that will enable high capacity, long haul Raman-amplified optical communication systems to enjoy commercial feasibility.
These, and other, drawbacks, limitations and problems associated with conventional optical communication systems are overcome by exemplary embodiments of the present invention, wherein the nonlinear effects which impact wideband, Raman-amplified optical communication systems are examined and system design parameters are adjusted to avoid limitations on system performance imposed by nonlinear effects. More specifically, Applicants have noted that a portion of the data carrying bandwidth is impacted by nonlinear effects to a significantly greater degree than the remainder of the data carrying bandwidth. This, in turn, results in a signal-to-noise ratio ceiling being imposed by the nonlinear portion of bandwidth on the entire data carrying bandwidth, beyond which ceiling the performance of the system degrades to an unacceptable level.
This problem is solved according to exemplary embodiments of the present invention by analyzing and adjusting system parameters so that the entire system SNR can be increased, e.g., on the order of 1 dB. For example, the so-called Q factor performance (described below) of a Raman-amplified optical communication system can be improved by allocating a predetermined bandwidth for communicating optical signals, dividing the predetermined bandwidth into wavelength channels, wherein a first plurality of the wavelength channels have a first spacing and a second plurality of the wavelength channels have a second spacing less than the first spacing, transmitting the optical signals via the wavelength channels; and amplifying the optical signals using Raman amplification, whereupon the first spacing associated with said first plurality of wavelength channels and the second spacing associated with the second plurality of wavelength channels equalizes the Q factors for the optical communication system.
According to another exemplary embodiment of the present invention, a method for communicating optical wave division multiplexed (WDM) signals includes the steps of: providing a first plurality of wavelength channels having a first spacing and a second plurality of said wavelength channels having a second spacing less than the first spacing; communicating optical WDM signals via the first plurality of wavelength channels using a first launch power; and communicating optical WDM signals via the second plurality of wavelength channels using a second launch power; wherein the at least one first launch power is greater than the at least one second launch power.
Even more generally, the present invention teaches that by making the channel spacing a function of the signal launch power, the variance in nonlinear effects associated with launch power tilt can be equalized. Launch power tilt appears as part of an effort to minimize SNR excursion.